This invention relates generally to magnetic tape record and/or reproduce apparatus and more particularly to a structure providing an improved head-to-tape interface in helical scan rotary magnetic head and cylindrical tape guide assemblies used, for example, in wideband video and data tape record and/or reproduce apparatus.
The rotary head segmented scan wideband tape record and/or reproduce apparatus is an exceedingly complex assembly. In helical scan tape record and/or reproduce apparatus of this kind, information is recorded on and reproduced from a magnetic tape by one or more magnetic heads rotated at a high velocity relative to the tape, which is transported at a much lower velocity along a helical path defined by a cylindrical drum tape guide structure. The heads and cylindrical drum tape guide structure usually are an assembly commonly referred to as a scanning assembly, or simply a scanner. The information is recorded on the tape in discrete parallel tracks that diagonally extend along the tape at an angle, typically of a few degrees relative to the length of the tape, so that the length of a recorded track greatly exceeds the tape width.
Rotary head helical scan tape record and/or reproduce apparatus employ a variety of different tape handling apparatus, rotary head handling apparatus and tape guide apparatus. Some use a single head to record and/or reproduce information relative to the tracks on the tape, while others use a plurality of two or more heads to do so. Regardless of the number of heads, usually they are mounted to rotate in a circular path as information is transferred in respect of the tape. The cylindrical drum tape guide structure is commonly formed of two axially displaced cylindrical drum sections. In a widely used helical scan rotary magnetic head and cylindrical tape guide assembly, the upper drum is mounted for rotation and the lower drum is mounted stationary, with the record and/or reproduce heads mounted to the upper rotatable drum to be rotated thereby. Other helical scan rotary magnetic head and cylindrical tape guide assemblies have stationary upper and lower drum sections, with the rotary heads mounted to a separate rotatable structure, such as a disk or third drum section, for rotation between the stationary upper and lower drum sections.
In helical scan tape record and/or reproduce apparatus, two recognized alternatives of guiding the tape helically about the cylindrical drum tape guide structure are commonly used, which are generally referred to as the "alpha" and the "omega" wrap apparatus. In the alpha wrap apparatus, the tape is introduced to the helical path about a cylindrical drum tape guide structure for scanning by a rotary head from one direction relative to the drum guide and is wrapped completely around the drum so that it exists in the opposite direction relative to the drum guide. It is referred to as alpha wrap apparatus for the reason that the helical tape path about the cylindrical drum tape guide structure generally conforms to the Greek symbol alpha when one views the tape path about the cylindrical drum in a direction along the axis of the cylindrical drum. In omega wrap apparatus, the tape is introduced to the helical path about the cylindrical drum tape guide structure along a path extending towards the drum in a direction generally radial to the cylindrical drum, passes around an entrance guide that directs the tape onto the surface of the drum, helically extends about the drum, and passes around an exit guide to be directed thereby away from the drum in a direction generally radial thereto. The tape path about the cylindrical drum tape guide generally conforms to the shape of the Greek symbol omega when it is viewed in a direction extending along the axis of the cylindrical drum.
In all of these helical scan magnetic tape record and/or reproduce apparatus, the tape is directed about the cylindrical drum tape guide along a helical path, with the tape exiting the path about the drum tape guide at a different position axially displaced along the drum surface relative to its position of entry to the path. In some of these apparatus, the tape follows a full-wrap helical path about the cylindrical drum tape guide of near 360.degree. . In other of these apparatus, the tape follows a path of much less than 360.degree. , a common path being near 180.degree. called "half-wrap". These helical scan magnetic tape record and/or reproduce apparatus have provided a practical means of achieving the high head-to-tape speeds necessary for recording and reproducing wideband data, such as television video information.
The parameter of the rotary head helical scan tape record and/or reproduce apparatus critical to high quality wideband signal transfers is the head-to-tape interface. This is particularly so in such apparatus for tape record formats having narrow tracks of recorded information, either with narrow or no record-free space or guard bands between them, recorded and reproduced at high relative head-to-tape speeds, such as in digital video tape record and/or reproduce apparatus constructed according to either of the well known D-1 or D-2 formats. The head-to-tape interface is determined to a considerable extent by the point of contact between the head and the tape, which must be of an optimum configuration for optimum signal transfers. Optimum head-to-tape interface is achieved by establishing a perfectly uniform and symmetric head tip penetration relative to the tape surface over the entire scan of the tape by the head, while the tape is guided accurately and stably about the cylindrical drum tape guide structure. More particularly, the tip of the head where signal transfers occur must engage the tape in a manner which maintains a consistent degree of tip penetration relative to the tape along the entire scan of tape to optimize the quality of the signal transfer. Unfortunately, the dynamics of guiding flexible tape about a cylindrical tape guide and rotating a head in contact with the guided flexible tape have prevented achieving perfectly uniform and symmetric head tip penetration along the entire scan of the tape.
As is known, in rotary head helical scan tape record and/or reproduce apparatus the head tip engages the tape such that a slight lift or "tent" is created in the tape at the point of contact with the head tip that displaces the tape from the surface of the cylindrical drum tape guide structure. It is desirable to support the tape at the head tip so that the profile of the tent remains uniform and symmetrical at the head tip throughout the scan of the tape by the head and so that the head tip engages the tent of the tape throughout the scan of the tape by the head. Too much lift or "tenting," such that the apex of the tape tent remains away from the portion of the tip of the head where signal transfers occur, results in a loss of signal output to or from the head, and a concomitant drop in the output signal-to-noise ratio (S/N). Changes in the configuration of the tent at the head tip also leads to a loss in signal output and drop in output signal-to-noise ratio.
Typically, rotary head helical scan tape record and/or reproduce apparatus are arranged to generate an air film or bearing between the surface of the cylindrical drum tape guide structure and the tape, which supports the tape off the surface of the tape guide to facilitate the control of the transport of the tape about the tape guide. In the more common rotary head helical scan tape record and/or reproduce apparatus, the air film is created by an arrangement of upper rotating and lower stationary cylindrical drum tape guide sections. The upper rotating drum section generates an air flow which is drawn between the surfaces of the two drum sections and the tape transported thereabout to form the air film that supports the tape over the surface of the tape guide structure. If the thickness of the air film is uniform across the width of the tape and over the length of tape supported over the surface of the tape guide, a uniform space is maintained between the tape and the surface of the tape guide. With such an air film, the tape tent profile will be uniform and symmetrical at the head tip throughout the head scan with the head tip engaging the tent in the desired manner.
In practice, however, an air film of perfectly uniform thickness is not maintained throughout the head scan. Commonly, the air film between the cylindrical drum tape guide structure and the tape in the region near the location where the head enters or begins the scan of the tape is thicker than it is in the region near the location where the head exits or ends the scan of the tape. This non-uniformity of air film thickness is caused principally by the loss of air from the air film established between the tape and surface of the cylindrical drum tape guide structure. Several other factors can lead to a non-uniform air film thickness, such as variations in tape tension, variations in pressure at the gap between the axially displaced sections of the cylindrical drum tape guide structure and at the windows provided in the tape guide structure for the magnetic heads, and variation in the distance from the head to the tape edge over the scan of the tape by the head.
Such variations singly or in combination contribute to changes in the degree of tip penetration relative to the tape as the head scans the tape, which results in an undesirable non-uniform tape tent profile. The composite tape tent profile obtained over the scan of the tape has a relatively thick air film and consequent low head tip penetration in the region near the location where the head enters the scan of the tape and a relatively thin air film and consequently greater head tip penetration in the region near the location where the head exits the scan of the tape. This variation in the thickness of the air film and concomitant variation in head tip penetration results in changes in the contour of the tape tent profile at the head-to-tape interface, which contribute significantly to losses in signal output and degradation of signal-to-noise ratio.
While a high degree of head tip penetration consistency has been less important in helical scan tape record and/or reproduce apparatus designed for analog signals, the newer helical scan tape record and/or reproduce apparatus designed for digital signals require higher data transfer rates and relative head-to-tape speeds to record more data per unit area of tape, and use thinner tapes for storage of more data on a reel of tape. With relatively thick tapes, moderate tape speed and slow head rotation speeds found in properly designed record and/or reproduce apparatus for analog signals, there is no detrimental deterioration in signal output and degradation of signal-to-noise ratio due to the presence of the typical variation in the thickness of the air film and resultant variation in head tip penetration that alters the contour of the tape tent profile at the head-to-tape interface. Moreover, after a "wear-in" period for the head and tape, it is found that the signal output for such record and/or reproduce apparatus actually improve for the "used" tape. There is, however, no corresponding resultant improvement experienced with the thinner tapes and the newer, faster record and/or reproduce apparatus designed for digital signals, such as digital video tape recorders constructed according to either of the well known D-1 or D-2 formats. While "wear-in" still occurs in these newer, faster record and/or reproduce apparatus, the resultant change in tip penetration relative to the position of the head in the scan of the tape can produce over time a tapered decreasing signal output level along a scan of the tape from a maximum signal level at the head scan position near the location where the head enters the scan of the tape to a minimum at the head scan position near the location where the head exits the scan of the tape.
To achieve a more uniform and symmetric head tip penetration relative to the tape surface over the entire scan of the tape by the head, while the tape is guided accurately and with stability about the cylindrical drum tape guide structure, some rotary head helical scan tape record and/or reproduce apparatus have employed a magnetic head and cylindrical tape guide assembly having non-coaxial stationary and rotatable cylindrical drum tape guide sections, with the axis of the stationary section displaced a small amount relative to the axis of rotation of the rotatable section in a direction towards the location where the head exits the scan of the tape. For the precise tape guidance required in aforedescribed record and/or reproduce apparatus for digital signals, however, it has not been possible to displace the stationary guide section a sufficient amount to remove all undesirable tapered decreasing signal output level without losing stability of tape guidance about the cylindrical drum tape guide structure. When the displacement of the stationary guide section becomes too large, the radial distance to the tape extending about the stationary guide section becomes larger than the radial distance to the tape extending about the rotatable section over a substantial length of the scan of the tape by the head. This creates forces that act on the tape as it is transported about the cylindrical drum tape guide structure, which drive the tape axially along the tape guide away from the tape edge guiding elements commonly provided on the stationary cylindrical drum tape guide section.
There exists, therefore, a significant need for a helical scan rotary magnetic head and cylindrical tape guide assembly which establishes a more uniform and symmetric head tip penetration relative to the tape surface over the entire scan of the tape by the head without risking the loss of stability and accuracy of tape guidance about the cylindrical tape guide. The present invention fulfills these needs and provides further related advantages.